The treatment of water is currently of constantly increasing importance. In addition to drinking water, especially in the chemical and pharmaceutical industries, high-purity process waters are required which must be prepared in a large quantity as inexpensively as possible. High-purity water, in addition, is especially also required in the semiconductor industry, for example for rinsing silicon wafers, in particular after etching processes. The purity requirements of the water are known to be particularly high in this sector.
It is known that the provision of ultrapure water can be achieved with a multistage process comprising a first stage in which the raw water is softened and/or already partially desalinated, a second stage in which the water from the first stage is further purified in a pressure-driven membrane separation process, and a third stage in which the water is finally substantially completely deionized, for example by electrodeionization. In addition, further process steps, in particular for eliminating organic impurities, can be further provided.
Water softening and/or desalination in the first stage generally proceed by use of one or more ion exchangers. For the softening, cation exchangers in the sodium form are preferably used, whereas for the desalination, combinations of cation and anion exchangers are customary. The total ionic load of the water to be treated can be markedly reduced already by such methods.
Membrane separation processes which come into consideration are, in particular, reverse osmosis and nanofiltration, optionally also in combination. If relatively large amounts of dissolved carbon dioxide are present in the raw water, this process sequence can be further supplemented by a degassing step before or after the membrane separation process.
If a high water yield is of importance, the concentrate from the membrane separation stage can be treated in a further additional membrane separation stage, wherein the resultant permeate generally, owing to its high electrical conductivity, cannot be directly transferred to a deionization step. Instead, it is customarily recirculated and added upstream of the membrane separation stage to the water to be treated.
Electrodeionization devices, in customary designs, always require a solution which takes up the ions that are separated off from the water to be treated and discharges them (concentrate) from the device. This solution flows through at least one concentrate chamber, and the water to be treated through at least one diluate chamber. A high ionic conductivity in the concentrate chambers in this case is known to be achieved, in particular, by the following:                (a) an addition, e.g. of neutral salts being formed,        (b) the concentrate being recirculated through the concentrate chambers, in such a manner that the ions that are separated off accumulate there or        (c) the concentrate chambers (as also the diluate chambers) are packed with ion-exchange resins.        
Alternatively, concentrate from an upstream membrane separation stage can also be fed into the concentrate chambers of an electrodeionization device. However, this must generally be worked up in an intermediate step, as disclosed by WO 2005/113120.
It could therefore be helpful to provide a technical solution for the multistage treatment of water, which, compared with known solutions, shall have a structure kept as simple as possible, in particular as concerns the deionization stage.